I don't think this is possible. I read this in the documentation. On the target backup repository, the backup copy job creates a backup chain in the forever forward incremental backup method. The target backup repository always contains only one active incremental backup chain. Restore points in the chain are rotated according to the retention policy.
I think the advantage of synthetic full is the ability to create a full backup on the repository and not have to transfer all the data to the remote repository.